1. Field of the Invention
The present invention relates to an alkaline storage battery, a hydrogen-absorbing alloy electrode, and a method for producing the same.
2. Description of Related Art
Nickel-metal hydride storage batteries practically made using hydrogen-absorbing alloys have the characteristics such as low environmental pollution and high energy density. The batteries are being widely used as electric sources of various cordless equipment and electronic equipment. Furthermore, for the use as power sources of electric vehicles, it is strongly expected to increase the output by improving the high-rate discharge characteristics.
As materials of negative electrodes for nickel-metal hydride storage batteries, generally used are MmNi5 (Mm denotes a mixture of rare earth elements) alloys having a crystal structure of CaCu5 type in which a part of Ni is replaced with a metal such as Co, Mn, Al or Cu.
In such hydrogen-absorbing alloy powders, an oxide or hydroxide is formed on the surface owing to spontaneous oxidation. Therefore, the initial charge and discharge capacity as an electrode lowers and activation is needed.
Moreover, the elements such as Mn and Al which perform an important part in hydrogen absorption/release equilibrium pressure of hydrogen-absorbing alloys and life of batteries readily dissolve into alkaline electrolyte and precipitate as an oxide or hydroxide. The dissolution and precipitation of these elements cause reduction in alloy capacity and deteriorate cycle life.
In addition, rare earth elements precipitate as hydroxides and cover the alloy surface. As a result, electrical conductivity decreases and charge and discharge characteristics at high-rate are insufficient.
As a conventional method for enhancing activation of hydrogen-absorbing alloys, JP-A-61-285658 discloses to immerse hydrogen-absorbing alloys in an alkaline aqueous solution. JP-A-9-7591 and JP-A-9-171821 disclose methods of immersing hydrogen-absorbing alloys in an alkaline aqueous solution and then in an acidic aqueous solution.
According to the former method of treating the hydrogen-absorbing alloys with an alkaline aqueous solution, the metal components such as Al and Mn and oxide films which are present on the surface of the hydrogen-absorbing alloy powder and which are soluble in an alkaline aqueous solution are dissolved. The dissolution forms on the surface of hydrogen-absorbing alloys a nickel-condensed layer (nickel-rich layer) which is higher in nickel concentration than other portions. In this way, it is attempted to improve activity of hydrogen-absorbing alloys.
According to the latter methods of treating the hydrogen-absorbing alloy with an alkaline aqueous solution and then with an acidic aqueous solution, an insulating hydroxide of low solubility (e.g., Mm(OH)3) produced on the alloy surface by the alkali treatment is dissolved in the acid solution and removed. In this way, it is attempted to improve the activation of the hydrogen-absorbing alloy and obtain a sufficient initial discharge capacity. In these methods, the hydrogen-absorbing alloy is dry ground and a drying step is provided between the steps of alkali treatment and acid treatment.
However, the formation of nickel-condensed layer on the surface of hydrogen-absorbing alloy or the removal of the hydroxide on the surface of the alloy according to the conventional techniques mentioned above cannot provide sufficient high-rate charge and discharge characteristics in the use for electric vehicles or electric tools which are demanded to have a high output.
The main object of the present invention is to provide an alkaline storage battery and hydrogen-absorbing alloy electrode capable of charging and discharging at high-rate by providing many pores on the surface of hydrogen-absorbing alloy to further improve the activity of the alloy in addition to the formation of a nickel-condensed layer on the surface of the hydrogen-absorbing alloy and the removal of hydroxide, and, besides, provide a method for producing the same.
For attaining the above object, in an electrode made using hydrogen-absorbing alloy powders comprising at least one rear earth element, nickel and at least one transition metal element as main components, the present invention uses as the hydrogen-absorbing alloy powders those having a structure of a nickel-condensed layer being exposed at the surface and having many pores.
The above hydrogen-absorbing alloy powders are preferably produced by a method which includes a first step of wet grinding a hydrogen-absorbing alloy, a second step of treating the resulting powder with an alkaline aqueous solution and a third step of treating the powder with an acidic aqueous solution which is carried out in succession to the second step, namely, without a drying step.